## Warning: package 'leaflet' was built under R version 3.3.2
## [1] "SpatialPolygonsDataFrame"
## attr(,"package")
## [1] "sp"
## [1] "GEOID10" "STUSPS10" "NAME10" "ALAND10" "AWATER10"
## [6] "INTPTLAT10" "INTPTLON10" "median_age" "Shape_Leng" "Shape_Area"